One of the most common mistakes many amateur poker players make is that they only bet when they have a strong hand. They assume that the way to win at poker is to wait for a premium hand and then pile their money into the pot, hoping some oblivious chap pays them off. In reality, if you only put significant money in the pot with your premium hands, if your opponents are anywhere near competent, they will essentially never pay you off.
Suppose your opponent raises to 3 big blinds out of her 100 big blind stack from middle position and only you call on the button. The flop comes. Your opponent bets 4 big blinds into the 7.5 big blind pot.
Some players only raise in this spot with exactly 6-5, 8-8, 7-7, 4-4, and 8-7. While there is nothing wrong with raising these premium hands, since it is nearly impossible for your opponent to have a better hand, she can easily fold her entire range, meaning you should never get value.
To solve this problem, you should bluff from time to time. The hands that are best to bluff with are those that also have some chance to improve to the best hand. In this case, the best bluffs are flush draws and straight draws. If your goal is to be balanced (as it often should be), you can actually raise with about two times as many semi-bluffs as value hands because many of your semi-bluffs will improve to premium hands on the turn.
So, in this situation, in addition to raising with your premium made hands, you should raise with many of your draws such as 10-9 and flush draws. If you are known to have a tight image, your opponent will play poorly by folding much too often (because she will incorrectly assume that you are only raising with premium hands). If your opponent is a calling station, that is fine too, because your premium hands will get paid off and your draws will complete by the river a decent amount of the time.
If you bet the flop and your opponent calls, it is usually wise to continue betting on the turn with most of your range. As on the flop, this will make you difficult to play against because your opponent will have no idea whether or not you have a premium hand or a semi-bluff. The same goes for the river.
It is important that you develop a strategy such that any action you take is not obviously strong or weak. If your opponents know your flop bets are always strong, you make their life easy. You win when your opponents make mistakes, so develop a strategy that will lead them to make as many mistakes as possible.